Cleaning efficiency testing method and apparatus for a filter in a filtering system

ABSTRACT

A method and an apparatus are provided for testing the efficiency of a cleaning procedure for a filter in a filtering system. After the cleaning procedure of the filter, the filter pores contain a fluid. The system is pressurized, and a decay in pressure over a predetermined time period is measured. Based on the pressure decay, it is determined whether or not the cleaning procedure has been effective; a pressure decay smaller than a predetermined threshold value indicates that the cleaning procedure has not been effective.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for testingthe efficiency of a cleaning procedure for a filter in a filteringsystem.

BACKGROUND ART

Sterile milk can be defined as milk free of micro-organisms which cangrow under prevailing storing conditions. Microfiltration is one of anumber of methods used for producing sterile milk. WO 98/57549 and U.S.Pat. No. 5,256,437 describe methods for the production of sterile milkusing microfiltration. According to these methods, the milk is passedthrough a filter through which only particles smaller than a certainsize can pass. This means that e.g. harmful microorganisms are filteredout. Filters used in microfiltration processes are often made of ceramicmaterial.

During the production of sterile milk in this manner, the filter needsto be cleaned after a certain amount of time. As a rule, the system doesnot enter a cleaning phase in the midst of a production cycle; this onlyhappens if the pressure that is required to maintain a constant flowthrough the filter exceeds a certain threshold value. This pressureincreases gradually during a production cycle, as the filter poresbecome more and more fouled (i.e. clogged with particles filtered outfrom the milk).

The cleaning procedure is commonly called CIP (Cleaning In Place) and isperformed by flushing the system with water with added detergents. Aproblem that sometimes occurs is that this cleaning fails; the cause ofthis might for example be a failure in the opening of a valve somewherein the system. When the filters have not been completely cleaned beforea production cycle starts, the cycle is shortened, since theabove-mentioned pressure rises too fast. This leads to product losses,since the system has to enter the cleaning phase sooner than calculated.

To avoid this it would be desirable to have some way of testing whetheror not the cleaning has been successful, before entering into aproduction cycle.

SUMMARY OF THE INVENTION

The object of the invention is to provide a solution to the previouslydescribed problems. This object is obtained by a method for testing theefficiency of a cleaning procedure for a filter in a filtering system,wherein the filter is provided with pores, comprising the steps of

performing the cleaning procedure for the filter,

wetting the filter,

pressurizing the filtering system,

measuring a pressure decay in the filtering system over a predeterminedtime period, and

determining, based on the pressure decay, whether or not the cleaningprocedure has been effective, wherein a pressure decay smaller than apredetermined threshold value indicates that the cleaning procedure hasnot been effective.

In a preferred embodiment, the filter is included in a system forproducing sterilized milk, preferably skim milk, and the cleaningprocedure is a Cleaning In Place (CIP) procedure, where the filter iscleaned with a fluid. At the end of the cleaning procedure, the fluid isdrained from the filtering system, leaving the filter pores soaked withthe fluid. The fluid is preferably but not necessarily water.

Advantageously, in case the cleaning procedure is found to benon-efficient, an additional cleaning procedure is performed for thefilter.

The above object is also obtained by an apparatus for testing theefficiency of a cleaning procedure for a filtering system, whichincludes a filter with pores. The apparatus comprises filterpressurizing means, a pressure sensor positioned to measure a pressurein the filtering system, and a controller which is operatively coupledto the filter pressurizing means and the pressure sensor. After thefiltering system has been subjected to the cleaning procedure, thecontroller actuates the filter pressurizing means so as to establish apressure over the filter, monitors an output from the pressure sensor soas to determine a decay in the pressure over the filter during apredetermined time, and estimates a qualitative result of the cleaningprocedure by comparing the determined pressure decay to predefinedreference data.

Advantageously, the filter pressurizing means comprise a gas supplyingdevice coupled to the filter, and first and second valves positioned atan inlet side and an outlet side, respectively, of the filter, whereinthe pressure sensor is positioned between these valves and wherein thecontroller is adapted to cause the gas supplying device to supply gas tothe filter as well as to close the first and second valves so as toestablish the pressure over the filter.

Moreover, the qualitative result estimated by the controller preferablyindicates whether the cleaning procedure has been efficient or not. Inthe latter case, the controller may be adapted to initiate an additionalcleaning procedure for the filtering system.

The method and apparatus according to the invention give the advantageof reducing the amount of production disturbances, since the risk ofhaving to interrupt a production cycle prematurely is considerablyreduced thanks to the invention. In turn, a more cost-efficientproduction is achieved.

Further scope of the applicability of the present invention will becomeapparent from the detailed description given hereinafter, However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawings,which are given by way of illustration only, and thus are not limitativeof the present invention, and in which:

FIG. 1 is a schematic view of a filtering system including an apparatusaccording to the invention.

FIG. 2 is a flow chart illustrating the method of the invention.

FIGS. 3 and 4 are diagrams of test results obtained during use of theinvention in a filtering system.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The filtering system in FIG. 1 includes a filter 1, a tank 2, a feedpump 3 and a circulation pump 4. On the pipes 5, 6 leading to and fromthe filter 1 there are two valves 7 a, 7 b. A test equipment 8, whichwill be described in more detail later, is connected to the system, asseen at 9 a, 9 b. Fluid from the tank 2 which enters the filter 1through the pipe 5 is divided into two fractions when passing thefilter, one being discharged at P and one at R. There are two pressuresensors 10, 11 in the system, one at each side of the filter.

The filtering system of FIG. 1 is particularly suited for the processingof skim milk; however other milk types are also possible.

When the system is used for sterilizing milk, the tank 2 contains milk,which by means of the feed pump 3 is fed to the filter 1 through thepipe 5. The fraction of the milk which only contains particles smallenough to pass through the filter 1 is discharged for possible furtherprocessing at P. This fraction is called permeate. Another fraction isdischarged at R, whereas the rest is recirculated through the pipe 6 forfurther processing through the filter 1. The fraction of the milk thatis discharged at R is called retentate. The circulation pump 4facilitates the recirculation of milk through the filter 1.

The pressure at P is measured by the pressure sensor 10, and thepressure at R is measured by the pressure sensor 11. The pressure overthe filter—which is called TMP (Trans Membrane Pressure)—is calculatedas the difference between these two by a TMP monitor 12. The TMP shouldnot exceed a certain threshold value during a production cycle. If thisvalue is exceeded it is an indication that the filter pores are tooclogged, wherein the production cycle will be interrupted by the TMPmonitor 12. Consequently, the remaining milk in the tank 2 cannot beprocessed during this cycle.

The system is cleaned using a CIP (Cleaning In Place) procedure. Whenthe system is in a cleaning phase, the tank 2 is filled with a suitablecleaning fluid. The cleaning fluid is passed through the system in thesame way as the milk, hence performing a cleaning procedure for thesystem and in particular removing the foulants in the filter pores. Inthe preferred embodiment, the cleaning procedure involves the followingsteps:

Initially, the system is flushed with water so as to remove any residualmilk.

Then, the system is cleaned with a basic fluid, such as sodium hydroxideor potassium hydroxide, possibly with the addition of tensides.Subsequently, water again flushes the system.

After this, the system is flushed with an acidic fluid, such as nitricacid. Finally, water is used to rinse the system.

However, the cleaning procedure itself forms no central part of theinvention and may be performed in other ways than above.

After the cleaning phase, the filter 1 is tested by the test equipment 8of FIG. 1 in accordance with the method presented in FIG. 2. The testequipment 8 includes a supply of gas 14, preferably a compressed airsupply, which is connected to the retentate side R of the filter 1. Apressure sensor 13 is positioned to measure the pressure at aforesaidretentate side R. A controller 15 is operatively coupled not only to thecompressed air supply 14 and the pressure sensor 13 but also to a firstvalve 7 a at the inlet side of the filter 1 as well as to a second valve7 b at the retentate side. The controller 15 has an optional operatorinterface 16, including e.g. a CRT or LCD monitor, a set of indicatorlamps or LEDs, a printer or plotter, a keyboard, a mouse, a joystick,etc.

The controller 15 also has a memory 17, such as a read/write memory(RAM, SRAM, etc), a read-only memory (ROM, PROM, EPROM, EEPROM), apermanent storage (magnetic or optical disk, etc), or any combinationthereof. The controller itself may conveniently be implemented by anycommercially available microprocessor (CPU), or another programmablelogic device such as a FPGA, together with program code stored in thememory 17. The controller 15 may alternatively be realized as anintegrated circuit, as discrete logic gates together with other analogand digital components, or in any combination of the above.

With reference to FIG. 2, a filter cleaning test method 20 according tothe preferred embodiment starts with the actual cleaning procedure 21,during which the filtering system, including the filter 1, is cleanedwith fluid from the tank 2 and, ultimately, flushed with water. In asubsequent step 22 the water is drained from the filtering system,leaving the pores of the filter 1 soaked with water. Then, in step 23,the controller 15 causes the compressed air supply 14 to supply airthrough pipes 9 a, 9 b to the filter 1. Moreover, the controller 15actuates the two valves 7 a, 7 b to assume their closed states, whereinthe filter 1 is sealed from the rest of the system and, consequently, ispressurized. An exemplary filter type may for instance be pressurized toabout 1600 mbar; this value may however vary widely between differentapplications and, in particular, different filter types.

Over time, air will diffuse through the filter pores, and the pressurewill drop accordingly. If the pores are clogged due to insufficientcleaning, there will only be a very small decay, since the diffusion ofair through the filter pores will be reduced. Consequently, this is anindication that the filter has not been cleaned properly. Thus, apressure decay measuring step 24 is initiated by resetting an internaltimer in the controller 15. An initial pressure value is obtained as amomentary output from the pressure sensor 13. Once the timer indicatesthat a predetermined time period has lapsed—the value of thispredetermined time period may conveniently be stored in the memory 17and may, for instance, be 120 seconds—the controller 15 again reads amomentary output value from the pressure sensor 13. The pressure decayis then calculated in the controller 15 as the difference between thetwo momentary output values from the pressure sensor 13.

In a step 25 the calculated pressure decay is compared to apredetermined threshold value, which too may be stored in the memory 17.If the calculated pressure decay is larger than (or, theoretically,equal to) the predetermined threshold value, the controller 15 considersthe decay to be normal and indicative of an efficient cleaning procedure21. The execution continues to a final step 28, where the remaining airis vented from the filtering system by opening the valves 7 a, 7 b and anew production cycle may commence. On the other hand, if the calculatedpressure decay is in fact less than the predetermined threshold value, astep 26 follows in which an operator may be alerted of the failed or atleast insufficient cleaning procedure 21 through the operator interface16. The operator will then be able to repeat the cleaning of the filter1, as indicated at 27 in FIG. 2. Alternatively, an additional cleaningprocedure 27 may be automatically initiated by the controller 15.

The additional cleaning procedure 27 may be performed in different ways.In the preferred embodiment the additional cleaning procedure 27 isrealized by repeating the normal Cleaning In Place (CIP) procedure 21 inits entirety.

Alternatively, however, the additional cleaning procedure 27 may involvethe same steps as the normal CIP procedure 21 but with either a shorteror a longer duration.

As a further alternative, only some of the steps of CIP 21 are performedduring the additional cleaning procedure 27.

According to yet another alternative, the concentration of the acidiccleaning fluid and the basic cleaning fluid, respectively, are eitherhigher or lower during the additional cleaning procedure 27 compared tothe normal CIP 21.

Still another alternative is to perform an extra step during theadditional cleaning procedure 27, for instance involving the use of anenzymatic cleaning fluid.

The test equipment 8 may advantageously be implemented by a commerciallyavailable integrity test equipment, such as PALLTRONIC FFE04 from PallProcess Filtration Limited, Europa House, Havant Street, Portsmouth PO13PD, England. Hitherto, such integrity test equipment has been usedsolely to verify that the filter element is intact, i.e. that there areno cracks or other defects in the filter that would allow largeparticles to pass through the filter which would in turn lead to anon-sterile product. In contrast, the present invention mayadvantageously make novel use of such equipment to test not for filterintegrity but for filter cleaning efficiency. It is emphasized that thetest equipment 8 may be realized by other means than aforesaid integritytest equipment.

FIG. 3 shows an example of how the pressure decay varies in a certainfiltering system. The system was in each test case 59–71 subjected to acleaning procedure after a production cycle. Then the system waspressurized to about 1600 mbar, and the pressure decay over 120 secondswas measured. In most of the cases, the pressure decay was about 50–60mbar, but in the test cases 62 and 63, the pressure decay was muchlower; 10–30 mbar. These two tests were performed after insufficientcleaning of the filter.

An improvement of the embodiment shown in FIGS. 1–3 will now bedescribed with reference to FIG. 4, which illustrates a long-termvariation in the pressure decay measured during step 24 of the filtercleaning test procedure 20 in the normal situation (i.e. when the filtercleaning is deemed to have been efficient, cf. step 28 in FIG. 2).

The diagram of FIG. 4 illustrates a series of pressure decay samplevalues as measured for a particular filtering system in step 24 of FIG.2 during a large number of production/cleaning cycles. As appears fromFIG. 4, following installation of the filter 1 (FIG. 1) in the system,the pressure decay measured in step 24 of FIG. 2 is more or lessconstant (at about 58 mbar) during the first few production/cleaningcycles. Then, at a slowly decreasing rate, the respective pressure decayvalues measured in step 24 start to fluctuate throughout theproduction/cleaning cycles. Eventually, after a large number ofproduction/cleaning cycles (e.g. at about cycle No. 75 in FIG. 4), thenormal pressure decay value measured in step 24 of FIG. 2 amounts toonly about 40 mbar, i.e. considerably lower than the initial valueslightly below 60 mbar as measured during the first couple of cycles.This long-term shift in normal pressure decay value is predominantly dueto the fact that no cleaning procedure will be absolutely perfect inreality (implying that insignificant amounts of milk residuals willremain in the filter 1 and, cumulatively, affect the normal pressuredecay value measured in step 24 of FIG. 2).

To handle the above situation and avoid confusing the long-term decreasein normal pressure decay with an abnormal pressure decay like tests No.62 and 63 in FIG. 3 (indicative of a failed cleaning procedure), thecleaning test controller 15 is adapted to perform a series ofcalibration pressure decay measurements when the filter 1 is initiallyinstalled in the filtering system. A reference value for normal pressuredecay is produced from this series of calibration measurements and isstored in memory 17. Moreover, a second reference value is calculatedfrom the first reference value and is also stored in memory 17,indicative of a limit below which the long-term decrease in normalpressure decay is regarded to be unacceptable in terms of risk ofconfusion with an abnormal pressure decay.

For instance, in the example of FIG. 4, the second reference value maybe set to 40 mbar, meaning that the cleaning test controller 15, atproduction/cleaning cycle No. 75, will notice that the pressure decaymeasured in step 24 of FIG. 2 is now below the acceptable limit asdefined by the second reference value and, thus, calls for action. Suchan action may for instance be that the cleaning test controller 15commands an additional cleaning procedure, in analogy with steps 26 and27 of FIG. 2, as has been described above. Such an additional cleaningprocedure will bring the normal pressure decay value back to the firstreference value (i.e. the initial pressure decay value calculated fromthe calibration measurements), or to a value which is close to thisfirst reference value. In other words, the reference value for normalpressure decay, as tested in step 25 of FIG. 2, is reset to its initialvalue as determined upon installation of the filter 1 in the filteringsystem. As an alternative, the cleaning test controller 15 may perform anew set of calibration measurements in addition to, or instead of, theadditional cleaning procedure so as to compensate for the long-termeffect described above.

The invention has mainly been described above with reference to apreferred embodiment. However, other embodiments than the one disclosedabove are equally possible within the scope of the invention, as definedby the appended patent claims. In particular, the exact values of thepressurization of the filter 1, the pressure decay measurement time andthe pressure decay threshold value(s) will have to be tuned to an actualapplication environment, as is readily realized by a skilled person.

1. A method for testing the efficiency of a cleaning procedure for afilter in a filtering system, said filter being provided with pores,comprising the steps of: performing said cleaning procedure for saidfilter, wetting said filter, pressurizing the filtering system,measuring a pressure decay in said filtering system over a predeterminedtime period, and determining, based on the pressure decay, whether ornot the cleaning procedure has been effective, wherein a pressure decaysmaller than a predetermined threshold value indicates that the cleaningprocedure has not been effective.
 2. The method as in claim 1, whereinsaid cleaning procedure involves use of a fluid in said filtering systemand wherein the step of wetting said filter is performed, uponcompletion of said cleaning procedure, by draining said fluid from saidfiltering system, while leaving the pores of said filter soaked withsaid fluid.
 3. The method according to claim 1 or 2, wherein said fluidcontains water.
 4. The method according to claim 1 or 2, wherein saidcleaning procedure is a Cleaning In Place (CIP) procedure.
 5. The methodaccording to claim 1 or 2, wherein the filter is used in a system forproducing sterilized milk.
 6. The method according to claim 1 or 2,comprising the further step of performing an additional cleaningprocedure for said filter, in case aforesaid cleaning procedure wasdetermined as not having been efficient.
 7. An apparatus for testing theefficiency of a cleaning procedure for a filtering system, saidfiltering system including a filter with pores, comprising: means forperforming a cleaning procedure for said filter; means for wetting thefilter; filter pressurizing means; a pressure sensor positioned tomeasure a pressure in said filtering system; and a controlleroperatively coupled to said filter pressurizing means and said pressuresensor; wherein said controller is adapted, after said filtering systemhas been subjected to said cleaning procedure, to actuate said filterpressurizing means so as to establish a pressure over said filter, tomonitor an output from said pressure sensor so as to determine a decayin the pressure over said filter during a predetermined time, and toestimate a qualitative result of said cleaning procedure by comparingthe determined pressure decay to predefine reference data.
 8. Theapparatus according to claim 7, wherein said filtering system furtherincludes a fluid container for feeding a fluid through said filterduring said cleaning procedure and wherein said controller is adapted,upon completion of said cleaning procedure, to drain said fluid fromsaid filter while leaving the pores thereof soaked with said fluid. 9.The apparatus according to claim 7 or 8, wherein said filterpressurizing means comprise a gas supplying device coupled to saidfilter and first and second valves positioned at an inlet side and anoutlet side, respectively, of said filter, and wherein said controlleris adapted to cause said gas supplying device to supply gas to saidfilter as well as to close said first and second valves so as toestablish said pressure over said filter.
 10. The apparatus according toclaim 7 or 8, wherein said pressure sensor is positioned between saidfirst and second valves.
 11. The apparatus according to claim 7 or 8,wherein said filtering system is included in a system for producingsterilized milk.
 12. The apparatus according to claim 7 or 8, whereinthe qualitative result estimated by said controller indicates whethersaid cleaning procedure has been efficient.
 13. The apparatus accordingto claim 7 or 8, wherein said controller is adapted, in case theestimated qualitative result indicates that said cleaning procedure hasnot been efficient, to initiate an additional cleaning procedure forsaid filtering system.
 14. The apparatus according to claim 7 or 8,wherein the pressure established by said filter pressurizing means oversaid filter is about 1600 mbar, said predetermined time is about 120seconds, and the pressure decay of said filter after having beensubjected to an efficient cleaning procedure is defined by saidpredefined reference data as 50–60 mbar, whereas the pressure decay ofsaid filter after having been subjected to a non-efficient cleaningprocedure is defined by said predefined reference data as 10–30 mbar.